cycle riding, and with some of the subjects the inability to urinate at the proper time made a sharply cut separation practically impossible. It was hoped that the collection of the urine could be divided into two periods, the first beginning when the subject rose in the morning and ending when the work experiment began, and the second covering the period of work. This separation was obtained in only two experiments, those of November 21 with H. L. H. and of March 27 with M. A. M. On the other hand, with M. A. M. specimens of urine were obtained at some time during practically every day of work, and determinations made of the total nitrogen. The results of these determinations are given in table 100, which shows that the nitrogen excretion per hour ranges from 0.06 to 0.77 gram, averaging not far from 0.4 gram per hour. Since the body-weight of this subject was approximately 65 kilograms, it can be seen that under the conditions of the experiments, which were usually part resting and part severe muscular work, the nitrogen excretion per kilogram of body-weight per minute was approximately 6 milligrams per hour per kilogram of body-weight. This is, if anything, TABLE 100.-Nitrogen in urine during experiments without food, with rest and muscular work.

1 Probably ate a light meal shortly before the collection of the urine.

2 Without load, with pedals rotated by motor. Times for beginning and ending approximate.

3 Without load, with pedals rotated by motor, 10h 37m to 11b 32m a.m.; work with resistance 11 32m a.m. to

12h 16m p.m.

4 Probably began work before 8h 30m a.m.

5 Work done in two periods, 8h 19m to 9h 21m a.m. and 10h 38m to 11h 30m a.m.

somewhat less than that found by Benedict and Carpenter in resting experiments with a large number of normal subjects." It is thus obvious that during the actual time of work there was not an excessive excretion of nitrogen per hour. This is further shown by the data for March 18 and April 8 with M. A. M., when the subject worked nearly the whole period of collection, the nitrogen excretion being about the same as in periods covering rest and work. On the other hand the figures throw absolutely no light upon the lag in the excretion of nitrogen following severe muscular work which has been observed by many investigators.

So much experimental evidence has already been accumulated to prove that protein cannot be the sole source of muscular work that no further attempt was made with our professional subject to collect the urine throughout the day, especially as this was difficult inasmuch as after the work was over each day he left the laboratory and the greater part of the time was uncontrolled as to diet. The data given in this table are interesting only in showing that even after the excessively severe work frequently done by the subject M. A. M. there was, on the whole, no great increase in the excretion of nitrogen from the body."

On the other hand it should be stated that the volumes of urine were for the most part very small. There was very free perspiration and hence the fluid excreted by the kidneys was reduced to a minimum. In one case but 50 c.c. of urine could be collected, while from 7 o'clock until 12 o'clock noon of another experimental day but 26 c.c. of urine were secured; in at least one instance (February 1) the subject was unable to pass any urine. It is thus obvious that with such fragmentary evidence as this no sweeping conclusions can be drawn, but that experiments planned primarily to study this problem are necessary in order to secure data that can be of material use. The values for nitrogen were secured primarily for the purpose of supplying data to be used in conjunction with the measurements of the gaseous exchange for computing the amounts of protein, fat, and carbohydrate katabolizeda computation frequently employed by many writers. On the other hand it should be stated that in all probability it is entirely erroneous to assume that the nitrogen excretion is coincidental with the metabolism of protein, as the excretion of abnormally large amounts of nitrogen are frequently observed after severe muscular work, showing that there is a distinct delay in the excretion of the decomposition products of protein. Although it has been pointed out that if any increase in nitrogen output takes place following work it usually appears two to three days afterwards, this is in the present series of experiments of little moment as the majority of the actual tests were carried out on consecutive days.

a Reported by Benedict and Joslin, Carnegie Institution of Washington Publication No. 176, 1912, p. 103. Fragmentary evidence bearing on this point has been secured in this laboratory on Marathon runners which shows that the total nitrogen output per hour during the severe work of running was small. See Higgins and Benedict, Am. Journ. Physiol., 1911, 28, p. 291.

V

MECHANICAL EFFICIENCY OF THE HUMAN BODY.

The relationship between the energy consumption and the output of work, which has been extensively studied with different machines, is a relationship which is likewise of great interest in the study of the mechanical efficiency of the human body. The experiments reported in this publication were primarily designed to furnish evidence with regard to the relationship between the total energy transformation and the external muscular work performed, and it is the purpose of this section of the report to discuss the experiments from this particular standpoint.

It may be desirable first to define what is meant by efficiency. Two terms are used by writers in this connection, but unfortunately often without sharp distinction. In this review of the literature on the subject, the term "gross efficiency" is used to designate the value obtained as a result of dividing the actual heat equivalent of the external effective muscular work by the total energy output of the man. This has frequently been designated by writers as "gross efficiency," "industrial efficiency," and "crude efficiency. When from the total output of the day is deducted the output for a period of corresponding length without external muscular work, the result gives the increase in the energy output which is caused by the work. Dividing the heat equivalent of the external muscular work done by the increase in the energy due to the work gives a value which may be designated as the "net efficiency." These two base-lines are those most commonly used by writers. In this publication other base-lines are considered and designations for these will appear in their regular places.

PREVIOUS STUDIES ON MECHANICAL EFFICIENCY OF THE HUMAN BODY.

Even the early literature gives evidence which can be used for the computation of the efficiency of the body as a machine, for in the experiments of Lavoisier and Séguin, which we have already cited, we find that for the performance of work equivalent to the lifting of 7.343 kilograms to 799 meters, i. e., 5,867 kilogrammeters, Lavoisier's subject required an increase in the oxygen consumption over and above resting of 36.8 liters. Assuming a respiratory quotient of approximately 0.87 (the average of the respiratory quotients obtained in over 100 experiments with our subject), the calorific equivalent of oxygen would be 4.887 calories, so that 36.8 liters of oxygen would correspond to 179.8 calories. The heat equivalent of 5,867 kilogrammeters would equal 13.8 calories; hence it appears that at the most about 7.7 per cent of the oxygen consumed over and above the amount consumed during rest was used for mechanical work, and about 4.4 per cent of the total amount of oxygen consumed. Lavoisier and Séguin also made observations on a man during digestion, and although the experiments are not well adapted for computing the efficiency, they are of interest in this connection. The total amount of oxygen consumed in an hour during work was 91.2 liters, an increase over resting of 53.6 liters. The subject performed 6,202 kilogrammeters of work, the calorific equivalent of which would be 14.6

See p. 33 of this report.

a The oxygen consumption per hour, resting, equalled 26.66 liters. In this report 1 calorie is considered as equal to 425 kilogrammeters, the value most used by the various investigators.

calories. Assuming a respiratory quotient of 0.90, the calorific equivalent of 91.2 liters of oxygen would be 449.1 calories and of 53.6 liters, 263.9 calories, so that it can be seen that of the excess of oxygen consumed, approximately 5.5 per cent was used for mechanical work, and of the total consumption of oxygen less than 3.3 per cent.

Following the experiments of Lavoisier and Séguin, no data from which the efficiency of the body can be computed were obtained until the experiments of Edward Smith." From the results obtained by Smith and by Despretz and Dulong, Helmholtz computed that of the total heat incidental to the performance of muscular work, one-fifth is used for mechanical work and four-fifths are given off in the form of heat, thus showing a gross efficiency of the body of 20 per cent.

Haughton, in an address before the British Medical Association at Oxford, calculated the average daily labor of a man to be 109,549 kilogrammeters, or the work necessary to raise a man of 150 pounds of weight to the height of 1 mile, and that the heat produced in 24 hours would be the equivalent of six times the average mechanical work performed by a laboring man. Thus he considers that the gross mechanical efficiency would be equal to 17 per cent.

Basing their computations upon the utilization by the body of 300 grams of carbon in the ascent of Mont Blanc, Dumas and Boussingault computed the efficiency of the body as 33 per cent.

TABLE 101.-Mechanical efficiency of the human body as computed by Hirn.

In 1857 a French physician in Colmar, Hirn,' attacked the problem of the mechanical efficiency of the body from the experimental standpoint. Considering Q as all the heat losses of the body, such as radiation, the warming of the expired gases, perspiration evaporated, etc., and T the work, he calculated that the total heat production of the body would be equal to Q+(T÷425). Hirn's results are given in abstract in table 101. There is obviously a great discrepancy between the calorific values of the oxygen

a Smith, Edward, Philosophical Transactions, 1859, 149, p. 681.

b Despretz, Ann. de Chim. et de Phys., ser. 2, 1824, 26, p. 337; Dulong, Ann. de Chim. et de Phys., ser. 3, 1841, 1, p. 440.

c Helmholtz, Proc. Royal Institution, 1861, 3, p. 347.

d Haughton, Relation of food to work, Dublin, 1868.

e Dumas and Boussingault, Essai de statique chimique des êtres organisés, 1844. Cited by Amar, Le rendement de la machine humaine, Paris, 1910, p. 21.

The first work of Hirn on the human motor was published in the Comptes rendus de la Soc. de physique de Colmar, 1857. See, also, Hirn, Théorie mécanique de la chaleur, Paris, 1857, 1.

consumed and the values of Q. Amar" shows that if we are to consider the values of Q as correctly measured calorimetrically, the efficiency agrees reasonably well with that of Helmholtz, who computes it as 20 per cent.

Although the classical experiment of Fick and Wislicenus in their ascent of the Faulhorn has frequently been cited as an indication of the fact that the work was not done by the disintegration of protein, it is a matter of special interest in this connection to note that these authors computed their mechanical efficiency as equivalent to 50 per cent.

Hirnargues that the effective work represents one-fourth of the chemical action in the muscles, namely, an efficiency of 25 per cent. Joule also contends that the efficiency is one-fourth, or 25 per cent, while in 1867 and 1868 Paul Bert computed that the values for English prisoners producing 260,000 kilogrammeters per day would be 34 per cent for the net efficiency and 21 per cent for the gross efficiency.

e

In 1887 Hanriot and Richet, working with their newly developed methods of studying the respiratory exchange and with a subject raising weights, assumed that muscular work is done by the consumption of glucose. As the net efficiency of their subject they found one-seventh, or 14.3 per cent, and concluded that the net efficiency of the body lies between 11.1 and 14.3 per cent.

In repeating some of Hirn's work on a wheel similar to the one used by him, Chauveau obtained results that were quite different. Computing the values from the amount of oxygen absorbed in excess of that absorbed during rest, the subject being without food in the stomach, Chauveau found a net efficiency averaging 14 per cent." Amar calls attention to the fact that there are marked differences in the calculated net efficiencies in two classes of experiments; thus, for going upstairs he found an efficiency as great as 15 to 30 per cent, but when the work was done exclusively with the muscles of the arm he found but 3 to 5 per cent. The criticisms regarding the length TABLE 102.-Results of experiments made by Laulanié with a constant amount of work.

of the experiments and the methods used in carrying out his researches have already been cited. A mathematical treatment of Chauveau's results. as to the efficiency under different conditions is admirably presented by Lefévre.

a Amar, loc. cit., p. 23.

b Fick and Wislicenus, Ann. des sci. nat., 1868, 10, p. 273.

Hirn, Revue scientifique, 1887, 39, p. 673.

d Joule, cited by Hanriot and Richet, Comptes rendus, 1887, 105, p. 76, who, in turn, cite from Fick, Mechanische Arbeit und Wärmeentwicklung bei der Muskelthätigkeit, Leipsic, 1881, p. 231.

e Bert, La machine humaine, 1867-68, 2, p. 50. Cited by Amar, loc. cit., p. 24.

Hanriot and Richet, Comptes rendus, 1887, 105, p. 76.

Cited by Amar, loc. cit., p. 25.

Lefevre (Chaleur animale et bioenergetique, Paris, 1911, p. 749). It should here be stated that a sharp distinction as to the effect of the various factors on efficiency has rarely been recognized by writers, hence Lefevre's abstract of Chauveau's work is of unusual interest. The variations in the percentage ascribed to the efficiencies by different writers is in large part explained by the lack of agreement on the basis of computation, a point that is clearly brought out by Lefévre.